A display device of the type mentioned in the opening paragraph is known from U.S. Pat. No. 4,113,360.
In said patent, a description is given of a display device comprising a first plate of a fluorescent material, in which, in operation, light is generated and trapped (so that this plate forms a light guide), a second plate which is situated at some distance from the first plate and, between said two plates, a movable element in the form of a membrane. By applying voltages to addressable electrodes on the first and second plates and to electrodes on the movable element, the movable element can be locally brought into contact with the first plate, or the contact can be interrupted. A transparent contact liquid is present on the contact surfaces. At locations where the movable element is in contact with the first plate, light is decoupled from said first plate. This enables an image to be represented. If the movable element is not in contact with the light guide, it is in contact with the second plate.
Contrast is a very important parameter by which the quality of a display can be judged. Light which is incident on the display screen of the display is reflected and reduces the visibility of, or blurs the image displayed on the display screen. Simplicity of design is also an important parameter.
It is an object of the invention to increase the contrast of the image displayed by the display part and yet to provide a relatively simple design.
To this end, the display device in accordance with a first aspect of the invention comprises a source for UV radiation, the movable element comprises means for scattering UV light, the display device comprises phosphor elements for emitting visible light at different colors when excited by UV radiation, and the phosphor elements comprise phosphor particles in an embedding material. Preferably, the phosphor particles have an average size a smaller than λ/π, most preferably between λ/6π and λ/2π.
By using UV excitable phosphor elements for generating different colors, it is possible to generate a color image. When the movable element is in contact with the light guide, the scattering centers in the movable element scatter the UV light out of the light guide, which UV light excites a phosphor element. The use of phosphors emitting different colors provides a color display device which uses a single type of light source. It is not necessary to use three different types of light sources (one for each color) or to use color filters for extracting colored light from white light, which inevitably results in appreciable losses of intensity.
By placing the phosphor particles in an embedding material (also called matrix), the scattering of all light including visible light by the phosphor element is reduced when using phosphor particles in the usual powder form. The difference in index of refraction between the phosphor particles and its surrounding medium is decreased, which reduces the scattering. In the preferred embodiment, the average size is small (smaller than λ/π, most preferably between λ/6π and λ/2π) which reduces the scattering. The UV absorption is roughly proportional to a/λ, whereas the scattering is much more strongly reduced as the size of the particles decreases. In the indicated range for the size of the phosphor particles, an efficient UV absorption is combined with a strongly reduced visible light scattering efficiency.
In a second aspect of the invention, the display device comprises a source for UV radiation, the movable element comprises particles for scattering light in an embedding material, and the display device comprises phosphor elements for emitting visible light at different colors when excited by UV radiation, the particles for scattering UV light in the embedding material having a scattering efficiency for the UV radiation emitted by the source, which is larger than the scattering efficiency for light in the visible range, preferably at least twice larger.
Within the framework of the invention the scattering efficiency of light in the visible range is considered to be the scattering efficiency for green light to which the human eye is most sensitive.
The inventors have realized that the nature of the particles for scattering may pose a problem in relation to contrast. Light incident on the scattering particles is in part scattered back to the viewer, which reduces the contrast. Scattering particles in a powder form or when placed on a surface scatter light in a wide range of frequencies. A second aspect of the invention provides scattering particles which have an appreciably larger scattering efficiency in the UV region than for visible light. Thus, the UV light is coupled out of the light guide by scattering in an efficient manner, and yet incident light is much less efficiently scattered.
In preferred embodiments, the average size a of the scattering particles in the movable element is smaller than λ/π, where λ is the wavelength of the UV radiation emitted by the source. For UV sources emitting UV light in lines, ‘the wavelength’ is the most prominent component of the spectrum emitted by the source. For UV sources emitting a continuous spectrum, ‘the wavelength’ is the peak of the continuous spectrum. For particles, the scattering efficiency (when in a matrix) is reduced sharply (roughly by a factor of (a/λ)k (k between 2 and 4)) for wavelengths larger than a. If e.g. λ=380 nm and a/λ<<1, then k=4 and the scattering efficiency for green light (λ=550 nm) is a factor of 4.4 less than that for UV. Preferably, at least 90% of the particles is smaller than λ/π. The most preferred range for the average particle size a lies between λ/6π and λ/2π. If the particles become smaller than λ/6π, the scattering efficiency for UV light decreases strongly. The reduced scattering can be overcome by increasing the number of particles (the density of the particles). This may lead, however, to agglomeration of the particles, the agglomerates forming, in effect, particles of a much larger size, for which the visible light scattering efficiency may be high.
In embodiments of the invention as regards its second aspect, the difference in index of refraction of the scattering particles and the embedding material is larger in UV than in visible light, preferably at least √2 larger. The larger the difference in index of refraction, the greater the scattering. By choosing scattering particles and embedding materials for which the difference in index of refraction is larger in UV than in visible light, the ratio of the scattering efficiencies in UV and in visible light is larger than one, thus increasing contrast.
To increase contrast and yet provide an even simpler design, the display device in accordance with a third aspect of the invention comprises a source for UV radiation, and the movable element comprises phosphor particles for emitting visible light at different colors when excited by UV radiation. Preferably, the phosphor particles are embedded in the material of the movable element. In preferred embodiments, the phosphor particles have an average size which is smaller than λ/π, where λ is the wavelength of the UV radiation emitted by the source. Preferably, the phosphor particles have an average size between λ/6π and λ/2π.
By providing the phosphor particles directly in or on the movable element, the design is simplified, because separate phosphor-comprising elements need no longer be used. Compared to embodiments in which the UV light is scattered in the movable element, exits this element and thereafter impinges on phosphor elements, no scattering particles are needed, which reduces the scattering of visible light, and more of the UV light can be used for exciting the phosphors, thus reducing the amount of phosphor material needed and thus further reducing scattered light by reducing the light scattered by the phosphor particles. By embedding the phosphor particles in the material of the movable element, scattering of visible light is further reduced. The indicated size ranges provide an even further reduced scattering of visible light.
The Figures are schematic and not drawn to scale, and, in general, like reference numerals refer to like parts.